Target Audience is a very important part of planning, writing and marketing your film. If you can't say who it is aimed at you are unlikely to be able to get funding for your film, it will also be more difficult to market your film.
The Audience and how they work:
Age: 64% of the audience is made up by people ranging from 7-34 years. So most films will be marketed and aimed towards that age range. However these people only make up 39% of the population. So the rest of the population rarely in comparison so it isn't worth aiming a film at them if you want to make money (which is what most films are made for)
Genre: The main genres out there are Sci-fi, horror, comedy, romance, action, thriller and film nor. Each genre attracts a certain kind of person and they will all expect certain things from films in a certain genre. For example people going to see a comedy will expect it to be funny whereas they won't expect this from a horror. Fans of one genre may not like other genres, like fans of Sci-fi are unlikely to also like romance. Genres can cross over as well to attract a larger audience.
Fame and shame: The fame of the cast or director can also effect who goes to see the film, some actors or directors may have a fan base who goes to see films they are in regardless of other things. Famous actors will be more expensive as well though so you have to take this into account. It can also work the other way round, if an actor has been involved in a scandal or something people may not want to go see their film.
Audience types: There are 4 main types of film watchers. Mainstream, mainstream plus, aficionados and buffs.
Mainstream people are the kind of person that only goes to see big blockbusters, films with a famous cast and a lot of advertising and generally are very mainstream. They use it as a way of escape so go to see it for enjoyment and don't expect to have different or unpopular views raised in the film
Mainstream plus people are similar to mainstream but may also watch slightly less popular films, also mainly for their enjoyment though.
Aficionados will watch a lot of films that aren't seen as big blockbusters and have a more unknown cast. These films may raise different or extreme ideas and they will go to see a film that challenges the way they think or makes them wonder about a matter. They will also see mainstream big blockbuster films.
Buffs usually go see films alone and possibly more than once. The films they see will usually be very unknown and they will have to seek themselves out themselves through magazines such as empire or the Internet. The films they see will raise unusual or taboo topics and will be much more "arty" and be in smaller less known cinemas. They are a small part of the audience so films made with the aim of making money will not be targeted at them.
Be sure you know who your target audience this is as it can affect, who comes to see it, how it is marketed, what kind of company will fund you, the cast that will want to be in it and why they see your film. If you want to make money it is probably best to aim it at mainstream and mainstream plus film watchers that are between 7-34. If you want people to think about your film and it isn't made to only make money it's best to aim your film at aficionados or buffs.